1. Field of the Invention
The present invention relates to an elastic wave device for use in a resonator or a band-pass filter. More specifically, the present invention relates to an elastic wave device that has a structure in which a LiNbO3 substrate is preferably used as a piezoelectric body and a dielectric layer, such as a SiO2 layer, is located on the piezoelectric body.
2. Description of the Related Art
Traditionally, elastic wave devices, such as boundary acoustic wave devices and surface acoustic wave devices, are used in band-pass filters of communication equipment, for example.
One example of this kind of boundary wave device is disclosed in WO98/52279 listed below. FIGS. 23 and 24 are a plan view that illustrates a boundary acoustic wave device described in WO98/52279 and a schematic partially cut-away front cross-sectional view that illustrates an enlarged main portion thereof, respectively.
A boundary acoustic wave device 1001 includes a LiNbO3 substrate 1002. An IDT electrode 1003 is disposed on the LiNbO3 substrate 1002. A polycrystalline silicon oxide film 1004 is arranged so as to cover the IDT electrode 1003. A polycrystalline silicon film 1005 is placed on the polycrystalline silicon oxide film 1004.
A boundary acoustic wave excited by the IDT electrode 1003 propagates while concentrating its energy in the polycrystalline silicon oxide film 1004 located between the LiNbO3 substrate 1002 and the polycrystalline silicon film 1005. Accordingly, a boundary acoustic wave device having a so-called three-medium structure in which a polycrystalline silicon film, a polycrystalline silicon oxide film, and a LiNbO3 substrate are placed in this order is formed.
WO98/52279 describes reliably confining a boundary acoustic wave excited by the IDT electrode 1003 in the polycrystalline silicon oxide film 1004 because the polycrystalline silicon film 1005 is placed on the polycrystalline silicon oxide film 1004.
WO2006/058579 listed below discloses a surface acoustic wave that includes a piezoelectric body, an electrode including a multilayer metal film in which a first layer and a second layer are placed in this order disposed on the piezoelectric body, and a dielectric layer covering the electrode. For this surface acoustic wave device, an acoustic impedance of the dielectric layer is Za, an acoustic impedance of the first layer of the electrode is smaller than 2Za, an acoustic impedance of the second layer of the electrode is higher than 2Za, and the percentage of the thickness of the second layer to the entire thickness of the multilayer structure of the first and second layers is in the range of 15%-85%. Here, an example of the material of the dielectric layer is SiO2, an example of the material of the first layer is Al, and an example of the material of the second layer is Pt.
In the case of a single-layer Al electrode, because the ratio between the acoustic impedance of Al and that of SiO2 of the dielectric layer is approximately one, its reflection coefficient cannot be high. In contrast to this, for an electrode having the above-described multilayer structure, its reflection coefficient can be high. In addition, because the Al layer can be thick, the resistance can be low, and thus the insertion loss can be small.
For the boundary acoustic wave device 1001 described in WO98/52279, a boundary acoustic wave propagates while concentrating its energy in the polycrystalline silicon oxide film 1004 between the LiNbO3 substrate 1002 and the polycrystalline silicon film 1005, but there is a problem in that a spurious component caused by a higher mode of the boundary acoustic wave occurs. It has been found that the magnitude of this spurious component caused by a higher mode reduces with a reduction in the thickness of the polycrystalline silicon oxide film 1004. However, there is a problem in that a reduction in thickness of the polycrystalline silicon oxide film 1004 leads to an increase in the absolute value of a temperature coefficient of resonant frequency (TCF) of the boundary acoustic wave device 1001.
For the boundary acoustic wave device 1001 described in WO98/52279, the acoustic velocity of a transversal wave of the polycrystalline silicon oxide film 1004 is lower than that of each of the polycrystalline silicon film 1005 and the LiNbO3 substrate 1002. Because the polycrystalline silicon oxide film 1004, in which an acoustic velocity of a transversal wave is low, is disposed between the polycrystalline silicon film 1005 and the LiNbO3 substrate 1002, in which an acoustic velocity of a transversal wave is high, a boundary acoustic wave excited by the IDT electrode 1003 can be reliably confined in the polycrystalline silicon oxide film 1004. Therefore, boundary acoustic waves at the fundamental mode and a higher mode propagate through the polycrystalline silicon oxide film 1004.
The fundamental mode is a mode at which a single anti-node is present in the polycrystalline silicon oxide film 1004, and it is so-called zeroth order mode. The higher mode is a mode at which a single node is present in the polycrystalline silicon oxide film 1004 and two anti-nodes having different displacement directions are present above or below the node, and it is a so-called first order mode.
A higher mode that has a plurality of nodes in the polycrystalline silicon oxide film 1004 may exist. However, responses at a higher mode other than the above-described higher mode are so small that they do not cause a problem.
The above-mentioned WO2006/058579 discloses merely the above specific multilayer structure as an electrode structure that can achieve an increased reflection coefficient and a reduced insertion loss in a surface acoustic wave device and does not describe any configuration that reduces a spurious component at a higher mode in a surface acoustic wave device.
As described above, for an elastic wave device in which a SiO2 film covers an IDT electrode, an increase in the thickness of the SiO2 film leads to an increase in a spurious component caused by a higher mode, and improvement in a frequency-temperature characteristic and reduction in a spurious component caused by a higher mode are a trade-off.